Which statement describes the knee's passive lock in midstance?

Which statement describes the knee's passive lock in midstance?

• A. Active quadriceps contraction maintains extension.
• B. Hamstrings actively prevent extension.
• C. Passive bone/ligament lock can maintain extension with minimal quadriceps activity.
• D. Plantarflexors stabilize the knee.

Answer: (C)

In midstance the knee can stay extended largely passively due to the joint’s setup and surrounding ligaments, not because the muscles must keep it straight. As the knee approaches full extension under weight bearing, the knee’s surfaces and the tension in ligaments—especially the structures that tighten at near-full extension—create a mechanical “lock.” This is the passive lock, a screw-home-type effect, where the bones and ligaments hold the knee in place so body weight can be supported with minimal muscle effort. Because the joint can lock itself mechanically, only slight quadriceps activity is needed to maintain extension and control small movements. The hamstrings, if they activated to pull the knee toward flexion, would work against this locking effect. Plantarflexors act at the ankle and don’t meaningfully produce or maintain the knee’s passive lock, so they aren’t the primary factor in stabilizing the knee in midstance.

In midstance the knee can stay extended largely passively due to the joint’s setup and surrounding ligaments, not because the muscles must keep it straight. As the knee approaches full extension under weight bearing, the knee’s surfaces and the tension in ligaments—especially the structures that tighten at near-full extension—create a mechanical “lock.” This is the passive lock, a screw-home-type effect, where the bones and ligaments hold the knee in place so body weight can be supported with minimal muscle effort.

Because the joint can lock itself mechanically, only slight quadriceps activity is needed to maintain extension and control small movements. The hamstrings, if they activated to pull the knee toward flexion, would work against this locking effect. Plantarflexors act at the ankle and don’t meaningfully produce or maintain the knee’s passive lock, so they aren’t the primary factor in stabilizing the knee in midstance.